Evolution of a Circumterrestrial Disk and
Formation of a Single Moon
Eiichiro Kokubo, Shigeru Ida, and Junichiro Makino
We investigate the evolution of a circumterrestrial disk of debris
generated by a giant impact on the earth and the characteristics of
the moon accreted from the disk by using high-resolution N-body
simulation.
We find that in most cases the disk evolution results in the formation
of a single large moon on a nearly non-inclined circular orbit
outside the Roche limit, which is consistent with the previous work
by Ida et al. (1997).
The efficiency of incorporation of disk material into a moon is
10-50%, which increases with the initial specific angular momentum
of the disk.
These results hardly depend on the initial condition of the disk as
long as the mass of the disk is 2 to 4 times the present lunar mass
and most mass of the disk exists inside the Roche limit.
The time scale of the disk evolution is determined mainly by the
surface density of the disk.
The evolution of the disk is summarized as follows:
The disk shrinks through the mutual collisions of disk particles.
Gravitational instability takes place and particle clumps grow
inside the Roche limit.
The clumps are elongated by the Kepler shear, which forms spiral
arms (non-axisymmetric structure).
Particles are transfered to the outside of the Roche limit due to the
gravitational torque exerted by the spiral arms.
When a spiral arm is extended beyond the Roche limit, the tip of the
spiral arm collapses to form a small moonlet.
The rapid accretion of these small moonlets forms a lunar seed.
The seed exclusively grows by sweeping out particles transfered over
the Roche limit.
When the moon becomes large enough to gravitationally dominate
the disk, it pushes the rest of the inner disk to the earth.
The formation time scale of the moon is of the order of a month.